Neoplasia is phenotypically an abnormality of differentiation. The cells do not mature into functional end cells but remain in the proliferative pool with a growth advantage over the normal clone. Induction of differentiation might afford a method to biologically modify the malignancy by inducing the neoplastic cells to differentiate to functionally mature, post-mitotic cells. Leukemia and preleukemia are excellent models to study biological respone modifiers, and the active vitamin D metabolite known as 1,25-dihydroxyvitamin D3 (1,25(OH)2D3 is one of the best characterized, physological inducers of differentiation. This seco-steroid is known to produce its diverse biological responses via interaction with specific receptors of a target organ. However, 1,25(OH)2D3 itself has been found to have a serious, dose-limiting toxicity of hypercalcemia. A fundamental premise of this application is that the 1,25(OH)2D3 receptors present in differing target tissues will have somewhat different structural requirements for optimal ligand interaction. Thus this NCCP proposal, which consists of four related programs, proposes to chemically synthesize new analogs of 1,25(OH)2D3 (including side chain, triene-modified, A-ring, and CD-ring analogs) and to carry out a compehensive evaluation of their biological properties. These analogs will be tested for their ability to induce differentiation and to inhibit proliferation of leukemia cell lines and neoplastic cells harvested from patients. The leukemic cells will be studied in vitro using a variety of clonogenic and differentiation assays and studied in vivo using nude mice and syngenic mice. A determination will be made of the in vivo toxicity (the consequence of chronic hypercalcemia) of the vitamin D analogs along with their ability to stimulate intestinal Ca2+ absorption and Ca2+ reabsorption from bone in vivo (potential contributors to hypercalcemia) and to affect the endogenous production of 1,25(OH)2D3. Also we propose to isolate the 1,25(OH)2D3 receptor from a responsive leukemic cell line to compare and contrast the ligand specificities with that of the intestinal receptor. From these studies we should be able to identify a vitamin D analog that will induce differentiation and inhibit proliferation of leukemia and preleukemia cells without causing hypercalcemia. Our approach will provide a model for selection of biological modifiers to neoplastic growth in general and several of the compounds may have activity on a wide spectrum of neoplastic and preneoplastic lesions.